1. Field of the Invention
The present invention relates to a document illuminating system for use in an image reader such as a digital copying machine or an image scanner to illuminate an object such as a document as well as to an image reader incorporating such a document illuminating system. It is applicable to a digital copying machine, a facsimile machine, a printer, and a complex machine of these machines.
2. Description of the Related Art
In recent years, LEDs (Light Emitting Diodes) have been well developed and highly bright LEDs are now available. LEDs offer such advantages as high longevity, high efficiency, high resistance to impact, and a mono-color light emission and will be expectedly adapted to various types of illumination including an illumination system of an image reader as a digital copying machine or an image scanner. Further, emission spectrum of a white LED covers a visible range of wavelength bands so that it is adoptable for a document illuminating system of a color image reader.
For this reason, various kinds of document illuminating systems using LEDs have been proposed. There is a well known technique to properly converge light beams with diffusion property from LEDs by a light guide plate or a reflective mirror in order to efficiently illuminate a document read area.
For example, Japanese Laid-open Patent Publication No. 2006-42016 (Reference 1) and No. 2006-295810 (Reference 2) disclose a document illumination system and an image reader to improve illumination performance of an illumination system by use of a reflective mirror. In both of the References, the position of the illumination system is diagonal relative to a document, the illumination system is configured to directly illuminate a document with a light component of a light beam from the LEDs at a small divergent angle to a direction orthogonal to a light emission face and to guide a light component at a large divergent angle to the document by use of a reflective mirror. Reference 1 exemplifies a structure with a reflective mirror including a curved face while Reference 2 exemplifies a structure with a reflective mirror including a planar face.
However, there is a problem in the References 1 and 2 that the diagonal position of the illumination system to the document leads to an increase in thickness and size of the illumination system in an orthogonal direction to the document. In order to decrease the size of the reader, a distance from a light source to the document need to be decreased, which makes it impossible to illuminate the document with an even luminance distribution. Furthermore, this illumination system is configured to illuminate the document from a single direction so that dark lines may occur in an image in reading an uneven document.
In addition, Reference 2 discloses an LED structure having a special covering element to cover an emission face, however, such a covering element faces a problem of high manufacture cost since general low-cost LEDs are not suitable for such a structure.
In order to deal with occurrence of dark lines in an image, Japanese Laid-open Patent Publication No. 2008-67276 (Reference 3) discloses a structure to illuminate an object with light from two illumination elements which oppose to each other (hereinafter, sometimes referred to as opposite illumination). This can prevent dark lines in an image of an uneven document illuminated with light from a single direction.
However, the two opposite illumination elements are each provided with light sources, which also results in increases in the size and manufacture costs of the illuminating system. From the sub-scan direction cross sectional (orthogonal to the document and including sub scan direction) drawing of the Reference 3, it is seen that the circuit board of light emitting elements extends diagonally downwards. Therefore, the document illuminating system of Reference 3 is increased in size in a height direction thereof. Moreover, since a normal line of the light emitting element directs to a direction of the document read area, it is difficult to secure a sufficient optical path length, leading to enlargement and uneven luminance of the illuminating system.
In view of solving the above problems, there is a demand for development of a document illuminating system in small thickness which can oppositely illuminates documents without occurrence of dark lines in an image.
Moreover, along with an increasing demand for a higher-speed, higher image quality image reader, it is required to develop a document illuminating system configured to guide light onto the light-receiving face of the image sensor in even luminance distribution in the sub scan direction for the following reasons.
A read line width of a CCD on a document corresponds to a size of the document in a main scan direction, and a position thereof in the sub scan direction may be shifted from a referential sub scan position due to an error in adjustment of such an optical element as a reflective mirror and an imaging lens or a movement of respective elements in reading a document from one end to the other in the main scan direction. Generally, the shift of a read line width may be about 0.2 to 1.0 mm in the sub scan direction relative to the optical axis of a read system.
Because of this, without illuminating a document with light amounts in even distribution, there will be an area insufficiently illuminated or an area over-illuminated in the document and a defective image (decrease in read accuracy) will occur. In order to prevent uneven illumination, a document area of about several mm in the sub scan direction need be illuminated in proper light amount.
Particularly, a light receiving portion (document read area) of a digital copying machine or an image scanner is very narrow in the sub scan direction, about 0.1 mm, for example. Because of this, the document read area need be illuminated in proper amount such that the center of a luminance distribution curve comes at the document read area; otherwise, luminance of the area is substantially decreased.
There is a demand for a document illuminating system for use in a digital copying machine or an image scanner to illuminate the document read area with even luminance even when the center of illumination shifts from the document read area, so that a wide area of the image sensor in the sub scan direction receives light in proper amount with even luminance distribution.
Preferably, the luminance distribution is to include, near a maximal value, an even portion in a width required to read a document with a fluctuation due to a mechanical error or the like added.
The even portion refers to a portion of the luminance distribution in which a data change rate of electric signals can fall within a practically allowable range by correcting the level of electric signals photo-electrically converted from an image by use of a signal processing circuit provided after the image sensor. For generating monochrome images, an input data change rate of about 30% is allowable by electrically correcting (amplifying) image signal values (gain adjustment). However, for generating color images, the input data change rate has to be about 12% or less since correction of color unbalance of three RGB colors need be taken into consideration rather than the correctable range of gain adjustment.
However, References 1 to 3 cannot achieve even luminance distribution and need to be improved.